1. Field of the Invention
The present invention relates generally to a light-emission drive/control system for solid-state optical devices, and more particularly to an optical record and reproduction apparatus for writing and/or reading information into or from an optical record carrier body by using a semiconductor laser device.
2. Description of the Related Art
Recently, an optical record/reproduction apparatus that stably stores an increased amount of information at higher recording density is becoming important more and more. In the recent manufacture of highly advanced digital equipment such as a computer system, an external storage unit employed therein is required to increase in the information storage capacity, as the computer tasks increase to meet a trend that the system deals with video movie images as well as static graphics images. Also, in the manufacture of a home-use or an industrial use video movie playback apparatus, as the quality of a reproduced image (the horizontal and vertical resolutions) increases, the required recording density of a record carrier body should increase to achieve an extended length of playback time. In either case, as the recording density increases, it will become more critical and less feasible to achieve higher operation reliability of the high-density record and reproduction apparatus.
The presently available optical record and reproduction apparatus employs a solid-state light-emitting device to read or reproduce information being stored in a rotatable round disk-shaped optical record carrier body. Such a record carrier body is generally called the "optical disk"; it may spell "optical disc" alternatively. A variety of types of optical disks are presently available, including an additional-write optical disk, phase-change (PC) type optical disks that can allow overwrite, a magneto-optical (MO) disks, and so forth. The solid-state light emitting device may be a semiconductor laser. The laser emits a light beam at a selected wavelength, which is projected onto the information record layer of a rotating optical disk. A reflected light from the optical disk is guided to enter an optical detector (photodetector). The photodetector detects the reflected light to generate an electrical signal indicative of reproduced information.
With the increasing needs for high performance and reliability of the optical record and reproduction apparatus (also referred to as the "optical disk drive apparatus," or "optical disk drive"), the high-accuracy drive of semiconductor laser devices has been demanded strongly. In particular, to attain an enhanced quality of the reproduced information such as an optical-disk image it is one of the most important subjects that the optical disk record/reproduction apparatus maintains the signal-to-noise ratio of a reproduced image as high as possible. The improvement of the signal-to-noise ratio of the reproduced image may affect the maintenance of high-density record performance of the optical disk, because such signal-to-noise ratio improvement leads to a decrease in the total storage amount of information signals to be written into the optical disk, causing a resultant effective recording density to increase.
To improve the signal-to-noise ratio of the optical disk record/reproduction apparatus, it is indispensable to suppress or prevent a noise (laser noise) from being generated in the semiconductor laser itself. In the employment of any one of the various types of optical disks indicated above, the laser noise is required to remain -120 dB/Hz or less in the relative-intensity-noise (RIN). Unfortunately, the simple use of the existing semiconductor laser may decrease the RIN causing it to range from -105 to -110 dB/Hz when the laser is 830 nanometers (nm) in oscillation wavelength and 40 mW in output power.
Conventionally, the "wide-band front-light automatic power control (APC)" method is known and utilized to suppress or eliminate the laser noise generation while allowing a presently available semiconductor laser to be used as a light source of the optical disk record/reproduction apparatus. The wide-band APC is described, for example, in H. Satoh et al., "Fast Laser Power Control for High density Optical Disk Systems," Optical Data Storage Conference, Feb. 25-26, 1991, at pp. 182-185/WA4. With the APC method, a part of a front light of a semiconductor laser being actually projected onto the optical disk in a read or playback mode is guided to a photodetector which converts it into a corresponding electrical detection signal. The detection signal may be used for the control of the light output of the semiconductor laser. Unlike a "rear-light" APC using a rear light emitted from the rear surface of the semiconductor laser as a monitor light, the front-light APC can attain an expanded controllable frequency band (control range) in its control loop, thus enabling the laser noise generation to decrease depending on the loop gain of the control system.
In the wide-band front-light APC, whether or not the maximum advantage can be taken therefrom may hang o the very point of how much the controllable range can be broadened with respect to the frequency band of a reproduced signal. An example of the feed-forward control technique providing such broad control range is disclosed in U.S. Pat. No. 5,097,473 by Toyoki Taguchi under the title of "Stabilized-Feedback Control Device For Semiconductor lasers."
However, the recent innovations are never satisfied with the improvement of the signal-to-noise ratio in the optical disk record/reproduction apparatus based on the laser noise reduction method, and still demands further improvement in accommodation for the future use. Particularly, the requirement to improve the signal-to-noise ratio in the high frequency band becomes very critical in the reproduction of higher precision image; on high-density optical disks. Under such a condition, a highly advanced laser control technique is demanded which can provide a more powerful laser noise reduction than ever.